The cell physiology of early development 



limited period after the removal of the gene. The situation of the factors studied by 

 Billingham and Medawar is obscure, since in the piebald guinea-pigs they studied 

 the originally colourless cells into which the factor passes probably possess the 

 same genotypic constitution as the coloured cells out of which it comes, the difference 

 between the cells being one which arises during differentiation rather than one of a 

 genetic nature. Beale (1954), who had studied these phenomena as closely as anyone, 

 has recently expressed a lack of satisfaction with the term 'plasmagene' for such 

 factors. Haldane (1954) is apparently of a similar opinion and has suggested calling 

 them 'mnemons'. For convenience in the present discussion, however, I shall continue 

 to refer to them as 'gene-initiated plasmagenes'. 



It will be noticed that the overwhelming bulk of the evidence for the existence 

 of plasmagenes comes from studies on micro-organisms. It might be, however, that 

 this is caused not by their rarity in other forms but by factors which make their 

 detection particularly difficult. It is clear, for instance, that if plasmagenes were to 

 play an important part in the differentiation of multi-cellular organisms, they could 

 not in general be capable of easy infective transmission from one cell to another, since 

 that would lead to an intermingling of different organs or types of tissue which should 

 remain separate. Thus we cannot expect to find many cases similar to that of Billing- 

 ham and Medawar, even if factors of an essentially similar nature were widespread. 

 It is necessary, therefore, to approach the matter to some extent from an a priori 

 point of view to try to determine how far plasmagene-like factors could fit in to the 

 mechanisms of differentiation in so far as we understand them at present. 



It is clear that the exogenous factors mentioned under group (1) above do not 

 come into the question. In the examples of the true plasmagenes mentioned in group 

 (2), the cytoplasmic determinant is a part of the general genetic constitution of the 

 organism and no more directly related to the differentiation of its various parts than 

 are the nuclear genes. It is, however, possible to imagine that the cytoplasm of the 

 egg of a given species might contain a number of different true plasmagenes localized 

 in various regions. Each region of the egg would then contain characteristic cyto- 

 plasmic factors endowed with genetic continuity which might determine the nature 

 of the organs which develop out of it. Such localized plasmagenes would, in fact, be 

 the same thing as used to be referred to at the beginning of this century as 'organ- 

 forming substances'. Now there is no doubt that in many eggs different regions of 

 the cytoplasm have different properties. The regions concerned are nowadays re- 

 ferred to as 'ooplasms', and opinion has rather moved against attributing their 

 properties to the presence of substances which are autonomous over against the 

 nucleus. 



The arguments which have swayed opinion against the old idea of organ-forming 

 substances are numerous. One is that the evidence suggests that the ooplasms are 

 only effective when they are able to interact with the nuclei; for instance, Seidel 

 (1929) has shown clearly that the cytoplasmic activation centre in the posterior of an 

 insect egg only becomes active when nuclei reach it. Spemann (1938) produced 

 similar evidence in connexion with the grey crescent ooplasm of the amphibian egg. 

 Again, as we saw in the case of the eye, discussed at the beginning of this paper, differ- 

 entiation from the egg to the final form takes place in a series of steps. It does not look 

 as though we are dealing merely with the sorting out of a number of factors which 



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